Electric double-layer capacitors, which enable rapid charging and discharging at a high current, have recently been put into practical use as electrochemical devices applicable to hybrid vehicles and electric vehicles.
Electric double-layer capacitors are provided with a polarizable electrode and an electrolyte solution, and driven on the principle that electric charges are stored in the electric double layer formed at the interface between the polarizable electrode and the electrolyte solution.
Electrolyte solutions often used in electrochemical devices such as electric double-layer capacitors are those obtained by dissolving a quaternary ammonium salt, for example, in an organic solvent such as a cyclic carbonate (e.g., propylene carbonate) or a nitrile compound (see Patent Literature 1).
For such electrolyte solutions, various methods are studied for improving the characteristics of electrochemical devices.
For example, in order to suppress degradation of withstand voltage and capacitance of electrochemical devices, the amounts of specific impurities in electrolyte solutions are reduced (for example, see Patent Literature documents 2 and 3). In order to improve the withstand voltage, a nonaqueous solvent is used including sulfolane or its derivative and a specific chain carbonate (for example, see Patent Literature 4). In order to improve the safety, an electrolyte solution is proposed including a specific electrolyte and a fluorine-containing organic solvent (for example, see Patent Literature 5).
The polarizable electrode of an electric double-layer capacitor is usually formed of activated carbon because of its large interface and excellent conductivity.
For example, Patent Literature 6 discloses an electrode comprising activated carbon for electric double-layer capacitors, the activated carbon being formed of a carbonized coconut shell and having a BET specific surface area of 2000 m2/g or larger and 2500 m2/g or smaller, an average pore size of 1.95 nm or greater and 2.20 nm or smaller, and a pore capacity of 0.05 cm3/g or greater and 0.15 cm3/g or smaller calculated by the Cranston-Inkley method among the pores having a diameter of 5.0 to 30.0 nm.